Triggered vacuum gap device with means for reducing the delay time to arc-over the main gap

ABSTRACT

Discloses a triggered vacuum gap device that comprises a pair of spaced-apart main electrodes defining a main gap therebetween and triggering means adjacent one of the main electrodes but relatively remote from the other for causing arc-over of said main gap upon activation of said triggering means. For reducing the time delay between activation of the triggering means and arc-over of the main gap when the relatively remote electrode is negative, means is provided for developing a magnetic field generally parallel to the electric field between the electrodes in the region of the triggering means.

tween and triggering means adjacent one of gap when the negative, means is provided for enerally parallel to the electric 3,188,514 6/1965 Cobine......................... 3,370,196 2/1968 Dorgelo et al. 3,384,772 5/1968 Rabinowitz.... 3,465,192 9/1969 Lafferty...................... Primary Examiner-Raymond F. Hossfeld Attorneys-4. Wesley l-laubner, William Freedman, Frank L.

Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACT: Discloses a triggered vacuum gap device that 313/162 comprises a pair of spaced-apart main electrodes definin ZOO/144B, 313/187, 313/198, 315/330 main gap therebe Int. the main electrodes but relatively remote from the other for causing arc-over of said main gap upon activation of said tri gering means. For reducing the time delay between activati of the triggering means and arc-over of the main relatively remote electrode is developing a magnetic field g field between the electrodes in the region of the triggering means.

Raymond L. Boxman Cambridge, Mass. App]. No. 808,617 [22] Filed Mar. 19, 1969 [45] Patented May 25, 1971 General Electric Company MEANS FOR REDUCING THE DELAY TIME TO ARC-OVER THE MAIN GAP 6 Claims, 1 Drawing Fig.

References Cited UNITED STATES PATENTS 3,127,536 3/1964 McLane....................... 3l3/153X United States Patent [72] Inventor [73] Assignee [54] TRIGGERED VACUUM GAP DEVICE WITH [50] Field of PULSE sauna:

'PATENIED M25197! 3,581,142

. PULSf SOURCE nvws/vro/e: RAYMOND L 'BOXMAN,

BY 6km ATTORNEY TRIGGERED VACUUM GAP DEVICE WITH MEANS FOR REDUCING THE DELAY TIME TO ARC-OVER THE MAIN GAP This invention relates toa triggered vacuum gap device and, more particularly, to means for reducing and making more consistent the time delay required toproduce an are between the spaced-apart main electrodes of such a device following activation of the triggering means.

The usual triggered vacuum gap device comprises a highly evacuated housing, a pair of main electrodes within the housing having a spaced-apart position for defining a main gap therebetween, and triggering means adjacent one of the main electrodes that is operable to inject a supply of charged conduction carriers into said main gap. A voltage is present between said main' electrodes, and entry of said conduction carriers into said main gap causes the voltage to produce an arc between said electrodes. An example of such a triggered vacuum gap deviceis shown in US. Pat. No. 3,087,092 to Lafferty, assigned to the assignee of the present invention.

The time elapsing between activation of the triggering means and arc-over of the main gap is referred to hereinafter as the delay time. Variations in this delay time are referred to hereinafter as the jitter time."

It has been found that the delay time in a typical triggered vacuum gap device is under 1 microsecond providing the particular main electrode adjacent which the triggering means is located is negative with respect to the other electrode at the time the triggering means is activated. But if the other electrode (referred to hereinafter as the relatively remote electrode) is the negative electrode at this instant, much longer average delay times are required for the main gap to arc-over. Also, the jitter time becomes much longer and more unpredictable when the remote electrode is the negative electrode.

An object of my invention is to provide a triggered vacuum gap device in which the total delay time and the jitter time are appreciably reduced under those conditions prevailing when the remote main electrode is the negative electrode of the main gap at the time of triggering.

In carrying out my invention in one form, I provide within a highly evacuated envelope a pair of main electrodes having a spaced-apart position defining a main gap therebetween. Adjacent one of said main electrodes but relatively remote from the other, I provide triggering means operable upon activation to inject a supply of charged conduction carriers into said main gap to initiate an are between said electrodes across said main gap. Means is provided for developing in the region where the charged conduction carriers enter said main gap a magnetic field that has its lines of force extending across said rnain gap generally parallel to the electric field between the main electrodes. This magnetic field is present when the charged carriers enter the main gap and has the effect of substantially reducing the delay time required to produce an areover between the main electrodes when the relatively remote electrode is negative with respect to the adjacent electrode.

For a better understanding of the invention reference maybe had to the following description taken in conjunction with the accompanying drawing wherein the single FIGURE is a side elevational view partly in section showing a triggered vacuum g ap device embodying one form of the invention.

Referring now to drawing, the triggered vacuum gap device shown therein comprises a highly evacuated envelope II and a pair of spaced apart main electrodes I2 and 14 within the envelope defining a main vacuum gap 15 therebetween. The interior of the envelope is evacuated to a pressure of 10 torr or lower. The illustrated envelope 11 comprises an inverted metal cup 16 having a lower end cap 17 suitably welded thereto at its mouth. At the upper end of cup 16, there is a central opening 19 and a tubular insulator 18 having its bore aligned with the opening. A metal end cap 20 is mounted atop the tubular insulator l8 and is joined thereto by a suitable Vacuumtight seal. Vacuumtight seals are also provided between the lower end of the insulator I8 and cup 16 and between cup 16 and lower end cap 17 to render the envelope Vacuumtight.

The main electrode 12 is a disc-shaped member of conductive material supported from the upper end cap 20 by means of a conductive rod 22. Main electrode 14 is a disc-shaped member of conductive material supported from lower end cap 17 by a plurality of circumferentially spaced conductive rods 24. When an arc is established between the main electrodes 12 and 14 across the main gap 15, as will soon be explained, current passes through the gap device via a path extending successively through the conductive parts 20, 22, l2, 14, 24 and I7.

For initiating an are between the main electrodes 12 and 14, triggering means 30 is provided adjacent a central opening 31 in the main electrode 14. This triggering means comprises a pair of aligned tubes 32 and 34 of a suitable metal, such as beryllium, and a thin annular disc of ceramic material 36 sandwiched between the tubes and normally electrically isolating the tubes from each other. Along the bore of ceramic disc 36 a trigger gap is formed having metal-to-ceramic interfaces located at opposite ends thereof. The upper metal tube 32 is electrically connected to main electrode 14 and the lower metal tube 34 is supported on a tubular insulator 38, which, in turn, is mounted on lower end cap 17. When a voltage pulse of sufficient amplitude is applied between tubes 32 and 34, a short arc is developed between the inner peripheries of the tubes 32 and 34 along the bore of ceramic disc 36. This arc immediately heats and vaporizes a small portion of the metal tubes 32, 34. The resulting vapor is immediately ionized by the arc and the ionized vapor is rapidly propagated, or injected, into the main gap 15 through opening 31, thus drastically reducing the dielectric strength of the gap and causing it to arc-over in response to the voltage then prevailing between the main electrodes 12 and 14. The ionized metal vapor particles that are injected into the gap are referred to hereinafter as charged conduction carriers.

As is well known, the lines of field distribution at the interface between a metal and ceramic body in intimate contact are highly favorable to a breakdown at such an interface. Such interfaces are present between ceramic disc 36 and metal rings 34 and 32, respectively. Accordingly, a relatively low voltage pulse applied across the trigger gap can initiate a discharge from one of these interfaces across the trigger gap.

Main electrodes 12 and 14 are made of a suitable metal, such as copper, that is substantially free of all gaseous impurities and impurities which upon decomposition will produce gases. Accordingly, the arc that is established between the main electrodes evolves no appreciable quantity of noncondensable gases from the main electrodes. This greatly aids the main gap 15 in recovering its dielectric strength immediately after a current zero is reached, assuming an alternating current. The metalic vapors that are produced by the are are condensed on the inner surface of the metal cup 16. This condensation occurs rapidly, and this permits the gap to build up its dielectric strength at a high rate when the current zero point is reached.

For applying a trigger pulse to the trigger gap at the desired instant, Iprovide a conventional pulse source 40 that has one of its terminals 41 connected to the upper trigger electrode 31 through conductive parts 17, 24 and I4 and its other terminal 42 connected to the lower trigger electrode 34 through a lead 44. Lead 44 extends through the lower end cap 17 via an insulator 45 which provides a hermetic seal between lead 44 and the end cap and electrically isolates parts 44 and 17.

In one embodiment of the invention, the triggered vacuum gap device is used for controlling an alternating current circuit comprising conductors 50 and 52 connected to the opposite end caps 20 and I7 of the gap device. It is desired to use the triggering means 30 for initiating an arc across the main gap irrespective of the then-prevailing polarity of the voltage applied to the main electrodes 12 and 14. It has been found that if the prevailing polarity is such that the lower electrode 14 is negative with respect to the upper electrode 12 then the main gap will arc-over extremely rapidly following arc-over, or activation, of the trigger gap. Typically, this delay time is less than 1 microsecond for any voltages across the main gap high enough to consistently produce arc-over thereof in response to arc-over of the trigger gap. But if the polarity is reversed, and the upper, or relatively remote, electrode 12 is negative at the time the trigger gap arcs over, then much longer delays, typically averaging about 6 or more times as long, will transpire between arc-over of the trigger gap and arc-over of the main gap. Moreover, this delay time will not be consistent, varying appreciably in length from one operation to the next. This variation in delay time is referred to as the jitter time." Jitter times of 4 or more microseconds were found to be quite common in tests made with the remote electrode negative at the time of triggering. it is to be understood that the time delay figures set forth in this paragraph were measured with no magnetic field present from the coil 60 (soon to be described).

1 have found that I can appreciably reduce the average delay time between activation of the trigger means 30 and arc-over of the main gap 15 by applying an axial magnetic field 56 to the main gap in the region where the charged conduction carriers enter the main gap from the triggering means and during the time of such entry. This axial magnetic field, l derive from a coil 60 wound around the cup 16. Cup 16 is a nonmagnetic material such as stainless steel, and the magnetic field developed by current through the coil is thus able quickly to penetrate into the main gap. in my tests described hereinafter the coil was energized froma separate source (not shown) with direct current of substantially constant magnitude so as to minimize the effects of eddy currents in the main electrodes.

Without the magnetic field from coil 60, it was found that an average of about 5.8 microseconds delay time elapsed between arc-over of the trigger gap and arc-over of the main gap when the upper electrode was negative. But with an axial magnetic field of 800 gauss developed by current through coil 60, it was found that this delay time was reduced to an average of about 3.1 microseconds. With a more intense magnetic field of 1,500 gauss, this average delay time was further reduced to about 2.1 microseconds. (1n the test circuit used for obtaining the data in this and the next paragraph, the voltage between the main electrodes was a voltage rising in ramp fashion at 1,000 volts per microsecond. The efiect of the magnetic field appears to be even more pronounced with a constant voltage applied between the main electrodes.)

The delay times became much more consistent and predictable as the axial magnetic field was increased. In this regard, the jitter time was reduced from about 4 microseconds with no axial magnetic field to about 1 microsecond with 1,500 gauss present.

Assuming no magnetic field, it has been found that a considerably higher voltage is required to produce arc-over of the main gap with the remote electrode 12 negative than with it positive. I have found, however, that the presence of my axial magnetic field appreciably reduces the arc-over voltage that is required with a remote electrode of negative polarity. More specifically, l have been able with a 1,500 gauss axial magnetic field to reduce the average negative polarity arc-over voltage to about one-half of that prevailing without the magnetic'field. In one specific triggered vacuum gap built substantially as shown and having a main gap length of one-half inch, 1 was able to reduce the negative polarity arc-over voltage from about 2,000 volts to about 1,000 volts.

The magnetic field 56 has been referred to hereinabove as "axial" since its lines of force 58 extend axially of the gap device in the crucial central region of the main gap immediately adjacent the trigger. The magnetic field may also be thought of as having its lines of force in this gap region extending generally parallel to the electric field between the main electrodes. This electric field (which is not shown) has its lines of force extending generally perpendicular to opposed surfaces of the two main electrodes in the central region of the electrodes.

In an effort to better understand the physical process by which the magnetic field reduces the delay times preceding arc-over of the main gap, photographs were taken of the main gap during the triggering interval. When no magnetic field was present, these photographs showed a diffuse glow in the main gap between the instant of triggering and main gap arc-over. This glow was in the central region of the gap and had a diameter of about twice that of the hole 31. When the magnetic field was present, however, the glow appeared to be confined to a region aligned with the hole 31 and of the same diameter as the hole. It gave the appearance of a columnated stream of the same diameter as the hole passing through the hole and across the main gap into impingement with the upper electrode. It is believed that the magnetic field constricts this plasma stream and thus increases the density of the ion current at the cathode surface, thus accelerating formation of a cathode spot on the upper electrode.

While 1 have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What 1 claim as new and desire to secure by Letters Patent of the United States:

1. A triggered vacuum gap device comprising:

a. an envelope evacuated to a pressure of 10 torr or lower,

b. a pair of main electrodes within said envelope having a spaced-apart position for defining a main gap therebetween, triggering means adjacent one of said electrodes and relatively remote from the other of said electrodes operable upon activation to inject a supply of charged conduction carriers into said main gap to initiate an are between said electrodes across said main gap, means for connecting said electrodes to a voltage source that can render said relatively remote electrode negative with respect to said one electrode at the instant said charged carriers enter said-main gap,

e. means for substantially reducing the delay time between activation of said triggering means and arc-over of said main gap when said relatively remote electrode is negative comprising: means for developing a magnetic field in the region where said charged carriers enter said main gap that has its lines of force extending generally parallel to the elec tric field between said electrodes, said magnetic field being present when said charged carriers enter said main gap,

f. said substantial reduction being a reduction as compared to the delay time required to initiate an are between said main electrodes when no magnetic field is present in said gap.

2. A triggered vacuum gap device as defined in claim 1 in which said magnetic field has an intensity of at least 500 gauss in the region where said charged carriers enter said main gap.

3. A triggered vacuum gap device as defined in claim 1 in which said magnetic field has an intensity of at least 1,000 gauss in the region where said charged carriers enter said main gap.

4. A triggered vacuum gap device as defined in claim 1 in which said magnetic field is of sufficient intensity to substantially reduce the voltage required to initiate an are between said mairi electrodes when said relatively remote electrode is negative as compared to the voltage required when no magnetic field is present in said gap.

5. The triggered vacuum gap device of claim 1 in which in the region where said charged carriers enter said main gap, the lines of force of said magnetic field extend completely across said gap generally parallel to said electric field.

6. A triggered vacuum gap device comprising:

a. an envelope evacuated to a pressure of 10" torr or lower,

b. a pair of main electrodes within said envelope having a spaced-apart position for defining a main gap therebetween,

'e. means for substantially reducing the time delay between 10 5 6 c. triggering means adjacent one of said electrodes and relative comprising:

tively remote from the other of said electrodes operable means for developing a magnetic field that has an effec-' "P activation Ej a ply 0f charged conduction tive component in theregion where said charged carcamel's into said l 8? mime between said riers enter said main gap extending parallel to the elecr electrodes across said mam gap,

tric field between said electrodes, said magnetic field being present when said charged carriers enter said mam gap, i. said substantial reduction being a reduction as compared to the time required to initiate an arc when no magnetic field is present in said gap;

d. means for connecting said electrodes to a voltage source that can render said relatively remote electrode negative with respect to said one electrode at the instant said charged carriers enter said main gap,

activation of said triggering means and arc-over of said main gap when said relatively remote electrode is nega- (u/f-ii) UP Patent No.

T 1. OFF K Datvd May 25, 1971 InvcnLor(s) Raymond L. Boxman It is; certified that error ilppuilffl in the and that. said Letters Claim 1, line 2, chan e "10- Claim 6, line 2, change "1.0

Putunt an; horn-1)) line 65, change above-idvntifiod n-atom: curructcd as; shown below:

" to l0- Signed and sealed this 1 9th day of October 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Acting Commissioner of Patents 

1. A triggered vacuum gap device comprising: a. an envelope evacuated to a pressure of 10 4 torr or lower, b. a pair of main electrodes within said envelope having a spaced-apart position for defining a main gap therebetween, c. triggering means adjacent one of said electrodes and relatively remote from the other Of said electrodes operable upon activation to inject a supply of charged conduction carriers into said main gap to initiate an arc between said electrodes across said main gap, d. means for connecting said electrodes to a voltage source that can render said relatively remote electrode negative with respect to said one electrode at the instant said charged carriers enter said main gap, e. means for substantially reducing the delay time between activation of said triggering means and arc-over of said main gap when said relatively remote electrode is negative comprising: means for developing a magnetic field in the region where said charged carriers enter said main gap that has its lines of force extending generally parallel to the electric field between said electrodes, said magnetic field being present when said charged carriers enter said main gap, f. said substantial reduction being a reduction as compared to the delay time required to initiate an arc between said main electrodes when no magnetic field is present in said gap.
 2. A triggered vacuum gap device as defined in claim 1 in which said magnetic field has an intensity of at least 500 gauss in the region where said charged carriers enter said main gap.
 3. A triggered vacuum gap device as defined in claim 1 in which said magnetic field has an intensity of at least 1,000 gauss in the region where said charged carriers enter said main gap.
 4. A triggered vacuum gap device as defined in claim 1 in which said magnetic field is of sufficient intensity to substantially reduce the voltage required to initiate an arc between said main electrodes when said relatively remote electrode is negative as compared to the voltage required when no magnetic field is present in said gap.
 5. The triggered vacuum gap device of claim 1 in which in the region where said charged carriers enter said main gap, the lines of force of said magnetic field extend completely across said gap generally parallel to said electric field.
 6. A triggered vacuum gap device comprising: a. an envelope evacuated to a pressure of 10 4 torr or lower, b. a pair of main electrodes within said envelope having a spaced-apart position for defining a main gap therebetween, c. triggering means adjacent one of said electrodes and relatively remote from the other of said electrodes operable upon activation to inject a supply of charged conduction carriers into said main gap to initiate an arc between said electrodes across said main gap, d. means for connecting said electrodes to a voltage source that can render said relatively remote electrode negative with respect to said one electrode at the instant said charged carriers enter said main gap, e. means for substantially reducing the time delay between activation of said triggering means and arc-over of said main gap when said relatively remote electrode is negative comprising: means for developing a magnetic field that has an effective component in the region where said charged carriers enter said main gap extending parallel to the electric field between said electrodes, said magnetic field being present when said charged carriers enter said main gap, f. said substantial reduction being a reduction as compared to the time required to initiate an arc when no magnetic field is present in said gap. 